Proton conducting polymers prepared by direct acid casting

ABSTRACT

A method for casting solid polymer electrolyte membranes comprising proton conducting polymers stable at temperatures in excess of 100° C. directly from acid solution. The invention further relates to the enhanced performance of these membranes with respect to conductivity. Particularly, the invention relates to the use of trifluoroacetic acid (TFA) as an acid solvent doped with H 3  PO 4  from which polybenzimiadazole (PBI) solid polymer electrolyte membranes may be cast.

FIELD OF THE INVENTION

The invention relates to a method for casting a solid polymerelectrolyte useful in fuel cells operated at elevated temperatures. Morespecifically, the invention is related to the use of a method of castingpolymer electrolyte membranes intended for use in fuel cells operatingon liquid fuels, the casting method involving casting the polymerelectrolyte membrane directly from an acid solution. The membranes castby this method demonstrate unexpectedly improved conductivity.

BACKGROUND OF THE INVENTION

In the past decade considerable effort has gone into the development andcharacterization of perfluorosulfonic acid polymer electrolytes such asNafion. These efforts have shown that polymer electrolyte membranes(PEM) offer a number of advantages over conventional electrolytes whenused in electrochemical devices such as fuel cells and waterelectrolyzers.

U.S. Pat. No. 5,525,436, entitled "Proton Conducting Polymers", thedisclosure of which is incorporated herein by reference, discloses theuse of polymer electrolyte membranes, for example polybenzimidazole(PBI) doped with phosphoric acid, which are capable of conductingprotons at temperatures of up to at least 200° C. These membranes,therefor, avoid prior art problems related to dehydration of themembrane. Further, disadvantages due to poisoning of the electrodecatalysts and fuel crossover are overcome by the novel polymerelectrolyte membranes disclosed in the patent. The preparation of themembranes of the patent involves first casting the membrane film from anappropriate solution, such as dimethyl acetamide (DMAc), and then dopingthe film with the desired acid constituent. Conductivity in the range offrom 0.01 to 0.04 S/cm for temperatures from 130° C.-190° C. and watervapor partial pressures up to 1 atmosphere were recorded for H₃ PO₄doped PBI films.

It has now been discovered that the conductivity of polymer electrolytemembranes of the type discussed above may be significantly andunexpectedly enhanced by preparation of the membrane from a solution ofthe doped polymer in an acid. For example, a PBI film doped with H₃ PO₄and prepared from trifluoroacetic acid (TFA) solution exhibitsconductivity measured at 0.04-0.08 S/cm, as compared to the lowerconductivity measured for PBI membranes cast from DMAc and subsequentlydoped.

It has further been discovered that the economics of membrane productioncan be reduced by casting the PBI membranes directly from a castingsolution containing H₃ PO₄ and including trifluoroacetic acid (TFA) as asolvent.

It is, therefore, an object of the subject invention to provide a methodfor casting a solid polymer electrolyte membrane which does not sufferfrom known problems associated with catalyst stability and activity, andwhich demonstrates enhanced conductivity.

It is another object of the invention to provide a casting solution fromwhich a solid polymer electrolyte membrane which is suitable for use indirect methanol fuel cells without exhibiting high methanol permeabilityresulting in loss in efficiency due to methanol crossover can beproduced by direct casting methods.

SUMMARY OF THE INVENTION

The subject invention relates to a method for casting solid polymerelectrolyte membranes comprising proton conducting polymers stable attemperatures in excess of 100° C. directly from acid solution. Theinvention further relates to the enhanced performance of these membraneswith respect to conductivity. Particularly, the invention relates to theuse of trifluoroacetic acid (TFA) as an acid solvent doped with H₃ PO₄from which polybenzimiadazole (PBI) solid polymer electrolyte membranesmay be cast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the conductivity as a function of water vaporactivity for DMAc cast PEMs;

FIG. 2 is a graph showing the conductivity as a function of water vaporactivity for PEMs comparable to that of FIG. 1, but cast from acidsolution according to the invention; and,

FIG. 3 is a graph showing the conductivity as a function of temperatureof films cast from acid solution and from DMAc.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to novel solid polymer electrolytes cast directlyfrom acid solution and to the significantly enhanced conductivity ofsuch solid polymer electrolytes. These membranes exhibit stability attemperatures up to at least 200° C. to overcome known catalyst stabilityand conductivity problems. Specifically, the invention relates topolybenzimidazole film doped with H₃ PO₄ cast directly fromtrifluoroacetic acid solution in combination with phosphoric acid. Theresulting membrane is suitable for use as a solid polymer electrolytemembrane in methanol-air fuel cells. These polymer films exhibitexcellent oxidative and thermal stability, these properties beingfurther enhanced by the acid nature of the polymer.

The subject solid polymer electrolyte membrane exhibits stable chemicaland electrical properties at temperatures well above 100° C. and up toat least 200° C., has good mechanical and film properties, demonstrateshigh proton conductivity and low fuel permeability, and is capable ofconducting protons with very low water activity.

The polymer used in the subject invention is preferablypolybenzimidazole (PBI), but may be any polymer known to the skilledartisan which is soluble in the TFA/H₃ PO₄ acid casting solution. Otherpolymers which may be suitable may be found in the disclosure of U.S.Pat. No. 5,525,436.

PBI's are known to have excellent oxidative and thermal stability, andare further stabilized by reaction with the doping acid in the acidcasting solution. Proton hopping between basic sites on the polymercompound and/or the imbibed free acid provides enhanced ionicconductivity. Further, the barrier properties of these films areenhanced due to the single phase morphology of the material, as comparedto the biphasic nature of the non-polar fluorocarbon/polar ionicmembranes. "Single phase morphology" refers to a microscopic continuousmatrix of a single material as contrasted to a two-phase system of apolar phase mixed with a non-polar phase.

In operation in a fuel cell utilizing a suitable fuel, such as methanol,the PBI polymer electrolyte membrane conducts protons from the fuelelectrode to the oxygen cathode. Carbon monoxide, present in fuels likereformed hydrogen, poisons the platinum catalyst commonly used in fuelcells. Liquid fuels like methanol produce even more severe poisoningeffects because carbon monoxide is an intermediate in the fuel oxidationprocess. However, at higher temperatures, approaching 200° C., the celloperation stimulates carbon monoxide oxidation to carbon dioxide,resulting in substantially enhanced catalyst activity in spite of thepoisoning effect of the carbon monoxide.

The invention will now be discussed with reference to the preferredembodiment of the invention, i.e. casting of the PBI film from TFA/H₃PO₄ solution. The following examples and related test data are intendedto be merely exemplary and in no way intended to be limitative of thesubject invention. Viable variations of the parameters presented, aswell as other aspects of the materials, their preparation and their use,will be apparent to those skilled in the art and are intended to becovered by this specification and the appended claims if falling withinthe meets and bounds thereof.

PEM Preparation

Films of polybenzimidazole were prepared by dissolving the polymer PBI(20 wt. %) in trifluoroacetic acid. This solution contained two phases.Phosphoric acid was then added to the solution, and the solution becomehomogenous. The solution was then filtered and cast on a clean glassplate using a Gardner knife. The film was heated at 140° C. in air forfifteen (15) minutes. It was removed from the glass plate and heated ina vacuum oven overnight at 90° C. to dry the film.

Conductivity Measurements

Conductivity measurements were made with a four point probe apparatus.In this apparatus, two platinum foil electrodes were clamped to the endof a 3 cm(L)×0.5 cm(W) polymer sample. These electrodes were used topass current through the sample. The magnitude and frequency of theapplied current were controlled using a PAR 173 potentiostat/galvanostatand a Wavetek 185 signal generator. Two platinum wires (0.5 mmdiameter), spaced 1 cm apart, were used to measure the voltage drop atthe center of the sample. Voltage measurements were made at three ormore current levels to ensure that the voltage-current behavior wasohmic. The current was applied at two different frequencies, 100 and1000 Hz. No frequency dependence was observed. The entire apparatus wascontained within a stainless steel vessel, which was placed inside anoven and connected to a gas manifold system so that the temperature,pressure and composition of the gas phase in contact with the samplecould be controlled. Measurements as a function of water partialpressure above 100° C. were performed by partially evacuating the celland then injecting sufficient liquid water via a septum to yield thedesired water partial pressure.

Samples were prepared from a solution of the polymer and phosphoric acidin trifluoroacetic acid (TFA). Initial measurements of the conductivityof the samples cast from trifluoroacetic acid indicated considerablyhigher values than those of the samples cast, for instance, fromdimethylacetamides (DMAc), as in U.S. Pat. No. 5,525,436, referred tohereinabove. The results from the conductivity measurements of thesesamples are presented in Table I, along with representative values forthe previous samples cast from DMAc. It is clear from the results inthis table that the films cast from TFA are significantly moreconductive than the films cast from DMAc. In addition, films of both 1mil and 4 mil thicknesses yielded similar conductivities. For both typesof samples, the conductivity increases with increasing acid content,with increasing water vapor activity and with increasing temperature.There are indications that the conductivity drops when the sample isheld at a high temperature overnight (compare the starred values tonon-starred values at similar temperatures and water vapor activities).However, even after these drops, the conductivity of the TFA samples isconsiderably higher than that of the DMAc samples.

                  TABLE I                                                         ______________________________________                                        Conductivity of Polybenzimidazole Samples Cast from                           TFA/H.sub.3 PO.sub.4 and H.sub.3 PO.sub.4 doped PBI Cast from DMAc            Casting H.sub.3 PO.sub.4 Content                                                                 Temperature                                                                             Water Vapor                                                                           Conductivity                             Solvent mol %      °C.                                                                              Activity                                                                              S/cm                                     ______________________________________                                        TFA     480        130       0.055   0.031                                    TFA     480        130       0.098   0.039                                    TFA     580        130       0.13    0.055                                    TFA, 4 mil                                                                            580        130       0.149   0.075                                    TFA     580        130       0.173   0.052                                    TFA     580        130       0.19    0.072                                    DMAc    501        130       0.1     0.01                                     DMAc    501        130       0.25    0.02                                     TFA     580        150       0.03    0.043*                                   TFA     580        150       0.05    0.052*                                   TFA     580        150       0.08    0.056                                    TFA     580        150       0.11    0.048                                    TFA     580        150       0.16    0.083                                    DMAc    501        150       0.12    0.022                                    DMAc    501        150       0.20    0.03                                     TFA, 4 mil                                                                            580        170       0.04    0.074                                    TFA     580        170       0.05    0.053                                    TFA     580        190       0.02    0.060*                                   TFA, 4 mil                                                                            580        195       0.017   0.068                                    TFA, 4 mil                                                                            580        195       0.024   0.078                                    TFA, 4 mil                                                                            580        195       0.025   0.049*                                   TFA, 4 mil                                                                            580        195       0.044   0.083                                    TFA, 4 mil                                                                            580        195       0.072   0.068*                                   DMAc    501        190       0.04    0.023                                    DMAc    501        190       0.12    0.040                                    ______________________________________                                         Samples are 1 mil thick unless otherwise noted                                *value obtained after sample had been at temperature for over 16 hours.  

Since the addition of a stronger acid (i.e. trifluoroacetic acid)appeared to improve the conductivity of the PBI/H₃ PO₄ system,additional samples were prepared containing two other strong acids,hexafluoroglutaric acid (HFGA) and squaric acid (SA). Conductivityresults for these samples are given in Table II and Table III. Incomparing these results to those given in Table I, it appears that notall acids yield higher conductivities. It is further noted, however,that in some instances (Table III particularly) where the mol % of H₃PO₄ was lowered due to the addition of another acid, while theconductivities are lower than those reported in Table I for TFA, theyare nonetheless as good as, or better than those reported in Table I forDMAc membranes.

                  TABLE II                                                        ______________________________________                                        Conductivity of Polybenzimidazole Samples Cast from                           TFA/H.sub.3 PO.sub.4 and HFGA                                                 H.sub.3 PO.sub.4 Content                                                                            Water Vapor                                             mol %    Temperature °C.                                                                     Activity  Conductivity S/cm                             ______________________________________                                        <365     130          0.165     0.008                                         <365     150          0.056     0.006                                         <365     170          0.026     0.005                                         365      130          0.151     0.017                                         365      150          0.071     0.017                                         450      130          0.110     0.031                                         ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Conductivity of Polybenzimidazole Samples Cast from                           TFA/H.sub.3 PO.sub.4 and Squaric Acid                                         H.sub.3 PO.sub.4 Content                                                                            Water Vapor                                             mol %    Temperature °C.                                                                     Activity  Conductivity S/cm                             ______________________________________                                        450      150          0.05      0.025                                         450      150          0.067     0.028                                         450      150          0.089     0.028*                                        450      150          0.094     0.030                                         450      150          0.143     0.034                                         450      190          0.017     0.021*                                        450      190          0.035     0.035*                                        450      190          0.058     0.033*                                        ______________________________________                                         *-value obtained after sample had been at temperature for over 16 hours. 

FIG. 1 is a graph illustrating conductivity results for PBI filmscontaining 501 mol % H₃ PO₄ cast from DMAc. FIG. 2 is a similar graphwhich demonstrates conductivity as a function of water vapor activityfor the preferred PBI film containing 580 mol % H₃ PO₄ cast from aTFA/H₃ PO₄ solution. FIG. 3 is a graph comparing the conductivity ofthese same films as a function of temperature where the upper curverepresents a film cast from acid solution according to the invention andthe lower curve represents the conductivity of a film cast from DMAc. Amarked increase in conductivity, between two (2) and five (5) fold, isshown.

Casting Films of Varying Thickness

The ability to cast PBI films from TFA with varying H₃ PO₄ concentrationwas also evaluated. In conducting this evaluation, PBI/TFA solutionswere prepared using different molecular weight PBI's and includingdifferent H₃ PO₄ ratios. Film 1 was cast in a special acid resistanthood, but with no protection to ensure slow TFA evaporation. Films 2-10were cast from the solutions in a glove box in the hood. Table IVreports the results of these casting exercised. These resultsdemonstrate the mechanical properties of the film, in terms offlexibility and inherent viscosity, η_(inh), which is defined asfollows:

    η.sub.inh =in(ηrel)/c

wherein η_(rel) -η/η₀, where η is flow time of the polymer solution, η₀is flow time of pure solvent and c is 0.5 gm/100 ml solvent.

                  TABLE IV                                                        ______________________________________                                        PBI/H.sub.2 PO.sub.4 Films Cast from TFA Solutions                                    η.sub.inh of PBI                                                                    Content of Thickness                                        Film No.                                                                              dl/gram.sup.a                                                                           H.sub.3 PO.sub.4 (M %)                                                                   (mil)   Properties                               ______________________________________                                        1       1.26      600        2.0     brittle                                  2       1.26      600        3.1     flexible                                 3       1.257     580        2.5     flexible                                 3-1     1.257     580        2.6     flexible                                 3-2     1.257     600        2.3     flexible                                 3-3     1.257     600        2.3     flexible                                 3-4     1.257     600        3.0     flexible                                 3-5     1.257     600        2.9     flexible                                 9       1.257     900        3.1     flexible                                 7-1     1.28      700        2.0     flexible                                 7-2     1.28      700        2.9     flexible                                 7-3     1.28      700        3.1     flexible                                 6-1     0.85      600        2.1     flexible                                 6-2     0.86      600        1.85    flexible                                 10-1    1.09      600        2.9     flexible                                 10-2    1.09      1000       3.0     flexible                                 ______________________________________                                         .sup.a The molecular weight of PBI for these inherent viscosities ranges      from about 20,000 to 30,000.                                             

As seen from the foregoing results, PBI films with good mechanicalproperties can be cast from PBI's of varying molecular weight, withvarying ratios of H₃ PO₄ and at varying thicknesses.

The invention has been described by way of example. Obviously,modifications and alterations will occur to others upon reading andunderstanding the preceding detailed description. It is intended thatthe invention be construed as including all such modifications andalterations insofar as they come within the scope of the appended claimsor the equivalents therefor.

We claim:
 1. A method for casting a solid polymer electrolyte membrane,comprising:preparing a solution of the polymer dissolved in an acidsolvent which further contains a doping acid; casting the solution toproduce a membrane; and drying the cast membrane.
 2. A method forcasting a solid polymer electrolyte membrane exhibiting enhancedconductivity in methanol/air fuel cell use comprising casting thepolymer from a solution of trifluoroacetic acid/H₃ PO₄.